Project Summary Nearly two-thirds of all cancer patients will receive radiation therapy (RT) during treatment, but radiation's curative potential is limited to local-regional disease in either the definitive or adjuvant setting. Locally advanced, unresectable disease represents 30-40% of pancreatic cancer patients at diagnosis and carries a 5- year survival rate of less than 1%. While RT is frequently used and highly effective in metastatic cancer for palliation of symptomatic disease, it has little impact on unirradiated distant sites and thus no impact on survival in the metastatic setting. Likewise in the setting of locally advanced regional disease, such as in pancreatic cancer, radiation is frequently studied in the neoadjuvant setting but with little apparent improvement in overall survival. We have found that radiation therapy activates a myeloid response in the tumor that suppresses adaptive immunity. By targeting the STING (STimulator of INterferon Genes) sensor with cyclic dinucleotides (CDN), we have found that we can prevent suppressive myeloid responses, and when CDN are combined with radiation therapy we can control aggressive murine tumors. We hypothesize that provision of STING ligands in combination with chemoradiation therapy removes the biomarkers of poor response to immunotherapy. We propose that ligation of STING in systemic monocytes that are recruited to tumors following radiation and/or chemotherapy prevents transition to a tumor-promoting environment and enhances tumor associated T cell responses. The specific aims of this study are to 1: Test the hypothesis that the multicomponent chemoradiation therapy for pancreatic cancer contains core components critical to generate adaptive tumor-specific immune response when combined with systemic STING ligand treatment; 2: Test the hypothesis that the enhanced tumor control provided by systemic STING ligand in combination with radiation therapy and/or chemotherapy results from recruitment of repolarized monocytes to the tumor that in turn promote adaptive immunity in the post-treatment tumor environment; 3: Test the hypothesis that immunotherapies targeting the myeloid compartment via STING ligands will be required for tumors with the biomarkers of poor preexisting immunity and high myeloid involvement. Our study design incorporates preclinical chemoradiation therapy of transplantable and spontaneous models of pancreatic cancer in immune competent mice using an advanced imaging and treatment platform. This proposal particularly focuses on control of both local-regional and distant disease to extend the curative potential of immunotherapy for cancer patients.